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Extending copper pipe run


Inca
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I have 14mm capped end copper pipes extended from the back of the downstairs shower room  into our attic.  I would like to extend 20 meters to the far side of the house to feed a w/c & handbasin and possibly an outside tap.  Should I upsize to 16mm pipe in the attic or stay with 14mm.   Thank you for reading.

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Pipe wall friction comes into it. The larger bore will have less resistance to flow and over a long length the resistance can be significant. Without getting out the theory book and cranking a few calculations I am not sure if the flow loss would be significant in this case. However I did something similar in our house when I re-run the pipework though a convoluted route to our new kitchen extension. Again I did not crank any calculations but just thought it would be prudent. Personally if the cost difference is not too much I would probably go for the larger bore.
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14MM will be fine to upsize you would still have a 14mm 'reducer' and flow rate would be defined by bore -vs- pressure -vs- resistance. What I have done is to introduce a second 16mm service after the water meter and put it on a pressure limiting valve as my Ballon was often letting by on the groupe securite.
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Lots of people take that mistaken view, look at the bore of any 15mm valve or compression fitting and you would say, I might as well use 10mm pipe as it is already restricted, the correct info was given a few postings back, the resistance to flow is proportional to the length of pipe run.

There are always good analogies to be had between water and electricity even they they are not good bedpartners [:)], saying that the restriction to flow is the tap or valve etc is like saying I might as well cable my ring main with one strand of a multi-core cable because the fuse wire which happily carries 13 amps is much smaller than that.

I have just had my lead water main replaced, I wanted to go up to 32mm as it will be serving 7 or 8 apartments but the tapping on the cast iron main was only for a 20mm pipe,  it would have involved shutting off the whole street to replace it with a 32mm one, by all accounts they have never heard of hot tapping around here. They also only stocked 20mm street counter assemblies and would have had to order a 32mm one leving me without water as the lead main was leaking.

After some discussion a compromise was reached, the meter is placed very close to the cast iron main, the connection between the two being made in 20mm PE pipe, then it is stepped up to 32 mm for the run to my dwelling thus greatly reducing the flow restriction like the electrical analogy and saving me the cost of a 32mm abonnement. The guy who does the works for the commune was sceptical so we did some flow tests before and after the work, what I have now is a tad less than te calculated flow rate for a 32 mm water main.

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You could simply regard it as a 16mm pipe with a very long 14mm reducer in it. There will be little or no appreciable gain as not matter how you stack it your available water will flow through the reducer to get to a larger pipe which will reduce slighty the potential velocity of the water through it gains vis a vis less frictional resistance will be lost at a) The fitting b) through velocity restriction in the 'reducer'
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To understand the difference between flow loss and a flow restriction I would suggest reading ‘Mechanics of Fluids’ by B.S.Massey. Published by Van Nostrand Reinhold (UK) Co. Ltd. It is a really good simple text which imparts all the essentials of fluid dynamics.
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If a)trial and error and b)engineering can be alternatively described as a) doing it and knowing what works b) Theorising...it's a wonder we dont all plumb our houses back to front really or waste money on expensive large tube upstream of restrictions....One day someone will come up with an alternative and start running large bore down to manifolds...Nah that would never catch on ....would it

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